I just finished doing the entire Halo 2 campaign on Legendary with a friend. It was in the MCC, we did it over the course of a couple days. We got all the skulls and did all the missions, and I got the achievement for the skulls, but I never got the achievement for completing Halo 2 on Legendary. The weird thing is that he did and I didn’t, even though we only played it together. Is there any way this could be fixed without doing it all over again?
I had a look at your service record and the only H2 levels Waypoint has registered as completed on Legendary (solo or co-op) for you are Cairo Station and The Great Journey. Unfortunately, it means you will have to play the remaining levels on Legendary again to get them to count for the achievement. Waypoint needs to register those levels as complete. Try and make sure you’re connected to Live at all times and it should work the second time around.